I've heard similar before mike. i think it is down to dimensional variations during manufacturing processes. These days some (v expensive!) props are made by CNC milling out of a solid block and the dimensional accuracy is perfect, though the grain pattern of the metal isn't. But afaik most are made (more cheaply) by casting then hand finishing and polishing, without any 3D computer check on the geometry and dimensions. Thus, there is bound to be some heat distortion of the blade shapes etc, which doesn't get corrected. Frexample people talk of "pitch" as if it were a constant value like you get on a nut or a bolt but of course each of the 5 blades has a different pitch caused by manufacturing inaccuracy and indeed each blade has a differnet pitch at each different point on the blade surface.

With all that scope for dimensional/geometrical error in the manufacturing it is perhaps not surprising that you get a 2kt or so variation.

That Sq58 I nearly bought in 2004 (1 yr old 2nd hand) that turned out to have hit rocks had the same issue. It had new props with the same nominal dimension props as the originals but they were made by someone other than teignbridge and that boat has always been 3kts down on top speed as a result.

Separately, how do your tillers work? Is there a tiller at the top of each rudder shaft, encased inside that triangular block thing at the top of the rudder shaft, ie is that triangle thing hollow? Or what?